Programmer - What is the actual meaning of seperation of content and presentation? explain with example

Programmer Question

What is the actual meaning of separation of content and presentation?

is it just mean to avoid inline css?

is it just mean if after writing html according to design then if then if we want to do any change in design then it should be only with css, no need to html

Can we really make any change in design from CSS only ?

• if we want to change the size of
  images then we will have to go to in
  HTML code
  if we wan to add one more line break in paragraph then again we will
  have to go to in HTML code


• If we want to add one more separator
  at some place then again we will have
  to go to in HTML code

Which X/HTML tag we should avoid to use to keep separation of content and presentation?

Is separation of content and presentation also helpful for accessibility/screen reader users?

Is separation of content and presentation also helpful for programmer/developer/designer ?